1. Field of the Invention
The present invention generally relates to an optical-pick-up device used for recording information on an optical memory and reproducing information from the optical memory, and particularly relates to a method of manufacturing the optical-pick-up device wherein an objective lens and a solid-immersion lens (hemisphere-shaped lens) are formed in a substrate by using a semiconductor manufacturing process. The device may be formed as a single integrated composite, or may be formed separately and connected together. The present invention also relates to an optical-pick-up assembly.
2. Description of the Related Art
FIG. 1 is an illustrative drawing showing a related-art optical-pick-up assembly used for an optical memory. In the figure, the optical-pick-up assembly includes a laser diode 1, a collimator lens 2, a polarized-light-beam splitter 3, a quarter-wave plate 4, an objective lens 5, an optical disc 6, a photodiode 7, and a convergence lens 8. The laser diode 1 emits a coherent laser beam having a predetermined wavelength. The collimator lens 2 applies optical correction to the laser beam so as to create a parallel beam. The objective lens 5 focuses the laser beam on a recording surface of the optical disc 6. The optical disc 6 has the recording surface on one side thereof facing the objective lens 5. The optical-pick-up assembly further includes other optical components for focus detection, track detection, etc., which are omitted from the figure for the sake of clarity.
In the configuration described above, the laser diode 1 emits the laser beam having a linear polarization, a direction of which is parallel to a surface of the sheet of paper bearing the figure. The laser beam is then optically corrected by the collimator lens 2 to become a parallel beam. The parallel laser beam passes through an optical isolator comprised of the polarized-light-beam splitter 3 and the quarter-wave plate 4. The passage of the laser beam through the optical isolator changes the linear polarization of the beam into a circular polarization. When being reflected by the recording surface of the optical disc 6, the laser beam changes a direction of the vibration, and becomes perpendicular to the surface of the sheet of paper after passing through the quarter-wave plate 4. The laser beam is then reflected by the polarized-light-beam splitter 3 to travel toward the photodiode 7, and is focused by the convergence lens 8 on the photodiode 7.
This configuration has limitations in that a size of the laser spot can only be as small as the wavelength of the laser light because a limit is imposed by optical diffraction. A size W of the laser spot is represented as: EQU W=proportional to L/sin .phi. (1)
wherein .phi. is an angle of a beam coming out from the objective lens. An NA (numerical aperture) of the lens is related to sin .phi. by the equation NA=sin .phi.. Further, L represents a wavelength of the light.
Because of the above-mentioned limitations, a hemisphere-shaped lens (solid-immersion lens) is arranged between the objective lens and the memory media, as is analogous to a liquid-immersion method in a microscopy, thereby stepping up an effective value of NA. This scheme is proposed by Kino with Stanford University. As shown in FIG. 2A, a solid-immersion lens 10a is positioned so close to the recording surface that a gap therebetween is smaller than the wavelength of the light. This configuration can draw on the fact that a spot size of the beam focused on the bottom surface of the lens is reciprocal to the refractive index of the lens.
With a refractive index n of the lens, the spot size is represented as: EQU W=proportional to L/(n sin .phi.) (2)
If the solid-immersion lens is more than half a sphere as shown as 10b in FIG. 2B, Snell's law is observed on the surface of the solid-immersion lens 10b, so that the spot size further becomes smaller. In this case, the spot size is represented as: EQU W=proportional to L/(n.sup.2 sin .phi.) (3)
If the lens is formed so as to have a thickness of r(1+1/n) (r: radius, n: refractive index), aberration can be suppressed to a relatively small level.
In the above configurations, the gap between the solid-immersion lens and the recording surface should be smaller than the wavelength of light, and should be as small as around 100 nm. To satisfy this requirement, an airborne head has been proposed based on aerodynamics (B. D. Terris, H. H. Mamin, and D. Rugar, "Near field optical data storage," Appl. Phys. Lett., 68, No.2, 141, 1996; U.S. Pat. No. 5,497,359).
FIG. 3 is an illustrative drawing showing such an airborne head.
The head of FIG. 3 has a solid-immersion lens 10c (refractive index=1.83) glued on an upper surface of a slider, and an objective lens 5 (NA=0.5) is positioned over the solid-immersion lens 10c at an appropriate distance. This configuration achieves a spot size of 360 nm when light having a wavelength of 830 nm is used.
Other documents disclosing use of a solid-immersion lens include the following. Japanese Laid-open Patent Application No.8-212579 discloses correcting spherical aberration when it is caused by a variation in the thickness of a lens or the thickness of a recording medium. Japanese Laid-open Patent Application No.8-221772 discloses a configuration for suppressing spherical aberration, and Japanese Laid-open Patent Application No.8-221790 discloses a configuration for suppressing coma aberration. In these documents, an airborne scheme is not used, and the solid-immersion lens and the objective lens are controlled by separate actuators.
With regard to manufacture of micro-lenses, the following documents may be referred to. Japanese Laid-open Patent Application No.6-194502 discloses a structure of a convex micro-lens and a method of manufacturing the same. Japanese Laid-open Patent Application No.6-208006 discloses an array of convex micro-lenses having a long focus distance. Japanese Laid-open Patent Application No.7-181303 teaches a method of manufacturing a micro-lens array in which each micro-lens has a convex surface on either side thereof. Japanese Laid-open Patent Application No.7-198906 discloses materials and devices used for micro-lenses of various shapes having a convex surface on either side thereof, and teaches a method of manufacturing such micro-lenses. Japanese Laid-open Patent Application No.7-244206 discloses a method of manufacturing a concave micro-lens.
For information about forming a curved surface by etching, the following documents may be referred to. Japanese Laid-open Patent Application No.5-173003 discloses applying dry-etching after forming a convex/concave curved surface on photoresist by use of scattered light or a diffuser. This document does not disclose making of a flat surface. Japanese Laid-open Patent Applications No.6-30090, No.7-281007, No.8-171003, and No.8-179299 disclose forming a concave surface by applying wet etching to a substrate, and pouring material having a high refractive index in order to create a flat-surface micro-lens for use in a liquid-crystal display.
Use of techniques disclosed in the above-identified documents does not help the configuration of FIG. 3 when this configuration fails to achieve a desired spot size. Such a failure is caused by a displacement of relative positions between an objective lens and a solid-immersion lens, and is exacerbated by a large NA of a combined lens comprised of the objective lens and the solid-immersion lens.
In other words, a desired spot size cannot be insured since it is difficult to maintain an accurate gap between the objective lens and the solid-immersion lens. This leads to a failure in achieving high-density recording/reproduction of information.
Further, some micro-lenses have a convex lens shape (i.e., a convex external surface). Such a shape prevents optical devices such as other types of lenses and light emitting/detecting devices from being overlaid even when there is a need to do so. This is because a substrate in this case does not have a flat surface that would make it easier to implement an overlaid structure.
Other micro-lenses have a flat external surface and are ensconced in a concave-surface recess formed in a substrate after such a recess is created by wet etching, or are implemented as a refractive-index distribution inside a substrate. It is difficult, however, to implement various structures within a single substrate because a manufacturing process becomes prohibitively complex, or because difficulties are encountered in creating a lens of a desired shape at an accurate position in either side of the substrate.
Accordingly, there is a need for an optical-pick-up device which insures a beam spot of a small size by maintaining accurate positioning of an objective lens and a solid-immersion lens, thereby achieving high-density writing and reading of information.
Further, there is a need for a method of manufacturing an optical-pick-up device which allows lenses to be readily created in a flat-surface substrate, and makes it possible to overlay other optical devices.